Guided by Japanese writings from an era of shoguns, an international
team of scientists today reported new evidence that an earthquake
of magnitude 9 struck the northwestern United States and southwestern
Canada three centuries ago. The findings are likely to affect
the region's precautions against future earthquakes and tsunamis.

Writing this week in the American
Geophysical Union's Journal of Geophysical Research, scientists
from Japan, Canada and the United States summarize old reports
of flooding and damage by a tsunami in 1700 on the Pacific coast
of Japan. With the aid of computer simulations, they conclude
this tsunami required a North American earthquake close to magnitude
9. Such an earthquake, in a few minutes, would release about
as much energy as the United States now consumes in a month.

The report's authors are Kenji
Satake, of the Geological Survey of Japan; Kelin Wang, of the
Geological Survey of Canada; and Brian Atwater, of the United
States Geological Survey based at the University of Washington
in Seattle.

The earthquake apparently ruptured
the full length of an enormous fault, the Cascadia subduction
zone, which extends more than 600 miles along the Pacific coast
from southern British Columbia to northern California.

Until the early 1980s, this
fault was thought benign by most scientists, said USGS's Atwater.
But then a swift series of discoveries in North America showed
that the fault produces earthquakes of magnitude 8 or larger
at irregular intervals averaging about 500 years. The most recent
of the earthquakes, dated approximately by radiocarbon methods,
occurred between 1680 and 1720.

These discoveries raised a
further question: Can the fault produce earthquakes of magnitude
9? Such a giant earthquake would produce minutes of low-frequency
shaking that might threaten tall buildings from Vancouver, British
Columbia, to northern California. A giant Cascadia earthquake
would also warp large areas of seafloor, thereby setting off
a train of ocean waves -- a tsunami -- that could prove destructive
even on the far side of the Pacific Ocean.

Such international concerns
motivated the research described today. "At issue for North
Americans," said Atwater, "is how to adjust building
codes and tsunami-evacuation plans to reduce losses of life and
property in the event of a future magnitude 9 earthquake in southern
British Columbia, Washington, Oregon and northern California."

Few scientists took that threat
in the Cascadia region seriously until 1996, when Japanese researchers,
in a letter to the international journal Nature, stunned their
North American colleagues by linking a tsunami in Japan to geologic
reports of an earthquake and tsunami at the Cascadia subduction
zone.

From the tsunami's arrival
time in Japan, the Japanese researchers assigned the earthquake
to the evening of Tuesday, January 26, 1700. In addition, from
preliminary estimates of the tsunami's height in Japan, they
guessed that the tsunami was too large to explain by a Cascadia
earthquake less than magnitude 9.

That guess was on target according
to today's report, which was published online by the American
Geophysical Union. The researchers begin by showing that the
1700 tsunami crested as much as 15 feet high in Japan. They then
use recent findings about the Cascadia subduction zone to relate
earthquake size to plausible areas of fault rupture and seafloor
displacement. Finally, they employ computer simulations of trans-Pacific
tsunamis to tune the estimates of earthquake size at Cascadia
to the estimated tsunami heights in Japan.

The findings, said Atwater,
justify precautions taken recently by engineers and emergency
personnel. Under construction standards adopted since 1996, engineers
have sought to design buildings to withstand giant earthquakes
in the northwestern United States. At the same time, state and
local officials have devised evacuation routes from areas believed
subject to a tsunami from a Cascadia earthquake of magnitude
9. In Canada, buildings in Vancouver and Victoria since 1985
have been built to resist stronger shaking from local earthquakes
than is expected from the next Cascadia earthquake; Canada's
2005 building code will explicitly include the hazard from the
subduction zone, said Wang.

Wang also noted that the giant
fault responsible for this earthquake is currently "locked,"
accumulating energy for a future destructive event. "Scientists
in the United States, Canada and Japan are carefully monitoring
the fault's activities using seismological and geodetic methods
and making comparisons with a similar fault in southwestern Japan,"
said Wang. "With a combination of a better understanding
of the previous earthquake and modern observations, we hope to
better define the potential rupture area of the future event."

Japanese scientist Satake noted
that since their first report in 1996 about the possible relation
between the Japanese documents and the American earthquake, the
Geological Surveys of the three countries began a joint project
on the Cascadia earthquake. "As a result of this international
collaboration, we have collected more evidence, made rigorous
interpretation of it, and have modeled the earthquake source
and tsunami propagation by using the latest techniques. Consequently,
we have confirmed that the 1700 earthquake was magnitude 9."